The present invention relates to thick-film circuit systems incorporating surface-mount technology (SMT).
Thick-film circuit systems are incorporated in many modern electronic systems. Typically, a thick-film circuit system includes a pre-fabricated substrate, one or more substantially horizontal layers of insulative dielectric material formed thereon, and one or more substantially horizontal layers of electrical conductors formed and interspersed between the layers of dielectrics. The layers of conductors are selectively electrically interconnected by way of metal vias which are formed and vertically routed through the dielectric layers which separate the conductor layers. The conductor layers thereby serve as electrical connections between various electrical components which may be formed and/or mounted over the pre-fabricated substrate. Such various electrical components may include, for example, pre-fabricated semiconductor integrated circuits (ICs), capacitors, inductors, and the like. In such a thick-film circuit system, the thick films which comprise the conductor layers may have, for example, thicknesses of anywhere from about 5 micrometers to about 30 micrometers. In addition, such a thick-film circuit system may even include, for example, ultra-thick thick films (UTTF) having thicknesses of up to about 130 micrometers.
Dielectric and conductor layers are typically formed over the substrate with a conventional screen printing technique. In such a technique, each individual layer is composed of its defining material elements and formed over the substrate by selectively screen printing the layer in a paste form over the substrate. Thereafter, the layer is then either merely dried or both dried and fired. During a typical drying step, the paste which is to form the new layer is exposed to an elevated temperature, for example, of 120xc2x0 C. During a typical firing step, the layer is exposed to very high temperatures of, for example, up to 850xc2x0 C. After firing, the new layer is cooled down to ambient or room temperature. Fabricating each layer with high temperatures in this way facilitates adhesion of the newly formed layer to the immediately underlying layer. It is apparent that since high-temperature fabrication occurs for each subsequent layer which is formed over the substrate, lower layers which have already been formed experience more high-temperature fabrication steps. Layers which experience excessive high-temperature steps may be susceptible to delamination or micro-cracking between layers. Thus, it is generally preferable to have as few layers on a substrate as possible to thereby reduce the number of required high-temperature steps.
Today, many thick-film circuit systems incorporate surface-mount technology (SMT), wherein pre-fabricated electrical components are soldered and thereby mounted to one or more conductors formed over the substrate. The firing of each of the conductor layers, however, often produces physical stress where the conductor layers and dielectric layers interface. Such stress at the interface of conductor layers and dielectric layers often causes undesirable delamination between the layers or produces undesirable micro-cracks in the layers at the interface which threaten to compromise the overall functional integrity of the thick-film circuit system. Furthermore, with regard to surface-mount technology, such delamination and micro-cracking is often exacerbated when a pre-fabricated electrical component is soldered to a conductor layer formed over a dielectric layer.
To avoid such delamination and micro-cracking problems, larger pre-fabricated components are typically soldered and mounted upon a conductor layer which is directly formed over the pre-fabricated substrate. The reason for this is because the pre-fabricated substrate is much tougher than a printed dielectric layer and so direct adhesion of a conductor to an underlying pre-fabricated substrate is typically better than the direct adhesion of a conductor to an underlying printed dielectric layer. As a result, any other conductor and dielectric layers carried on the substrate must be pre-formed and circuitously routed such that the substrate space that will ultimately be taken up by the subsequently-soldered electrical component will be unoccupied to accommodate the component. As a result, to properly accommodate the component, the density of printed conductors about each soldered component is necessarily increased and/or additional dielectric and conductor layers must be added to the substrate. Thus, the routing and printing schemes for conductor and dielectric layers around the component space must necessarily be more complex to conserve lateral substrate space. Also, since additional conductor and dielectric layers may be required to make up for the lost space to be occupied by the component, such additional layers dictate that additional high-temperature firing steps will be experienced by lower layers.
Thus, there is a present need in the art for improving the adhesion of printed conductors to underlying layers of printed dielectric over a substrate in a thick-film circuit system. If such adhesion were improved, alternating layers of conductors and dielectrics could then be formed over the substrate without having to provide lateral space for a pre-fabricated electrical component to be soldered and mounted to a conductor directly formed over the substrate. That is, the component could then instead be soldered to an upper or top conductor layer even though that same conductor layer is directly formed over a printed dielectric layer. In this way, the component need not necessarily be soldered only to a conductor formed immediately over an insulative substrate. As a result, lateral space upon the substrate would thereby be conserved by primarily routing conductor and dielectric layers underneath the component. Such would also reduce routing complexity and provide more flexibility as to the number of conductor and dielectric layers formed over the substrate.
The present invention provides a multi-layer conductor system with improved adhesion to printed thick-film dielectrics. According to the most basic embodiment of the invention, the multi-layer conductor system includes a base layer having an electrically insulative top portion comprising alumina, an electrically conductive intermediate layer formed on the top portion of the base layer, and an electrically conductive top layer formed on the intermediate layer. In this basic embodiment, the intermediate layer comprises alumina and a precious metal alloy having silver and a precious metal other than silver. Within the intermediate layer, the amount by weight of the precious metal alloy is greater than the amount by weight of the alumina, and the amount by weight of silver in the precious metal alloy is greater than the amount by weight of the other precious metal. The other precious metal within the intermediate layer is preferably selected from the group consisting of platinum and palladium. The metallic constituent of the top layer comprises silver. Preferably, the top layer is either essentially pure silver or a silver alloy. Such silver alloy is more particularly a precious metal alloy containing principally silver and lesser amounts of other precious metals. The term precious metal generally refers to gold, silver, platinum, and palladium.
Preferably, the difference between the percentage weight of silver in the precious metal of the top layer and the percentage weight of silver in the precious metal alloy of the intermediate layer is not more than approximately 5. In this way, each of the base layer, the intermediate layer, and the top layer are each characterized by a different amount of shrinkage with temperature such that the amount of shrinkage of the intermediate layer is between that of the top layer and the base layer. An advantage of this minimizing of difference in silver content is that it prevents silver diffusion, which can occur when there is a large difference in silver content between two layers and which results in a weak metal-depleted zone between the two layers.
According to preferred embodiments of the present invention, the intermediate layer has a percentage weight of alumina of up to approximately 10. Furthermore, in such embodiments, the difference between the percentage weight of silver in the precious metal of the top layer and the percentage weight of silver in the precious metal alloy of the intermediate layer is preferably not more than approximately 2.